The present invention relates to a uniaxially high-oriented polyethylene naphthalate film for polarizing plates and, in particular, to a film suitable for a polarizing plate which is excellent in heat resistance and weather resistance. More particularly, the present invention relates to a polyethylene naphthalate film useful as a polarizing plate, which is stretched uniaxially with high orientation, contains a dichroic dyestuff, and shows an in-plane birefringence of more than 0.110 after the film is stretched and subjected to heatset treatment.
Uniaxially stretched polyethylene terephthalate films containing a dichroic dyestuff have conventionally been used as polarizing plates having good mechanical properties, electrical properties and heat resistance.
A uniaxially stretched polyethylene terephthalate film, however, has been found to have various defects as it has increasingly been used for wide purposes, and an improvement has been demanded. For example, when a uniaxially stretched polyethylene terephthalate film containing a dichroic dyestuff is used as a polarizing plate, the dye which is capable of enhancing the degree of polarization is specified, and if the orientation of one direction is increased to solve this problem, the film is likely to be torn. In addition, since the film is poor in weather resistance, the film is unsuitable to be used outdoors.
In place of such polyethylene terephthalate films, polyethylene naphthalate films have been proposed. For example, Japanese Patent Laying-Open (KOKAI) No. 50-45877 (1975) discloses a biaxially oriented polyethylene 2,6-naphthalate film having a Young's modulus in the longitudinal direction of not less than 51,000 kg/cm.sup.2, and a Young's modulus in the transverse direction of not less than 68,000 kg/cm.sup.2, and which is useful as an electrical insulating material, a base material for magnetic recording tape. Japanese Patent Publication No. 56-19012 (1981) discloses a magnetic recording tape comprising a magnetic layer formed on the surface of a biaxially oriented polyethylene-2,6-naphthalate film which has a Young's modulus in the longitudinal direction of not less than 51,000 kg/cm.sup.2 and a Young's modulus in the transverse direction of not less than 68,000 kg/cm.sup.2 Japanese Patent Publication No. 48-29541 (1973) discloses a method of manufacturing a magnetic recording tape having a large tensile strength in the liongitudinal direction and a good dimensional stability, comprising a biaxially stretched polyethylene naphthalate film and a mixture of magnetic particles and a binder coated on the surface thereof.
However, no suggestion has ever been made as to the use of a uniaxially stretched polyethylene naphthalate film as a polarizing plate.
It is an object of the present invention to provide a film for polarizing plates which is superior in weather resistance, tear strength and heat resistance to a stretched polyethylene terephthalate film, and which produces no eduction of a low-molecular weight polymer such as an oligomer.
As a result of researches on how to achieve this aim, the present inventors have found that it is very difficult to obtain a film for polarizing plates having the above-described properties by using a polyethylene terephthalate film but it is enabled by using a polyethylene naphthalate film which is uniaxially highly oriented more than a predetermined value, and the present invention has been achieved on the basis of this finding.